In a trunking site of a trunked radio system, each base station of a group of base stations provides a channel (e.g., a distinct transmit/receive frequency pair or timeslots on a distinct transmit/receive frequency pair) with which the base station may communicate with a population of subscriber radios. During normal operation, one of the base stations of the trunking site (referred to below as a “control channel base station”) provides a broadcast control channel, while the other base stations provide bearer (traffic) channels. Although the control channel base station may be changed from time to time, only one base station at a time is typically assigned to be the control channel base station.
Once registered with a trunking site, a subscriber radio continuously monitors the control channel in order to receive regularly-transmitted subscriber control packets from the control channel base station. In addition, the subscriber radio may request communication on a specific talkgroup over the control channel. When such a request is granted, the control channel base station provides the subscriber radio with the frequencies (transmit and receive) and possibly the timeslot(s) corresponding to the bearer channel assigned to the talkgroup. The subscriber radio may then switch to that bearer channel. When the subscriber radio's inclusion in the talkgroup is completed, the subscriber radio returns to monitoring the control channel broadcast by the control channel base station.
In a trunked radio system that implements a “simulcast” trunking site, the group of base stations that provides the control and bearer channels is essentially replicated one or more times, where each instantiation of a base station group may be referred to as being included in a “sub-site.” Each control and bearer channel provided by corresponding base stations in the multiple sub-sites is carried on an identical frequency pair, and the corresponding base stations across the sub-sites for each channel transmit identical data packets substantially synchronously. This typically results in constructive interference of the data packets received by the subscriber radios (i.e., the power is summed at the subscriber radio). With such constructive interference, the subscriber radio is oblivious as to whether a single base station has transmitted a data packet or multiple corresponding base stations have synchronously transmitted the same data packet. Accordingly, to the subscriber radio, transmissions from corresponding base stations of a simulcast trunking site appear to be transmissions from a single base station of a non-simulcast trunking site, although the received signals from a simulcast trunking site likely appear to be of higher power.
In both non-simulcast and simulcast trunked radio systems, a subscriber radio that is monitoring a control channel expects to receive subscriber control packets from the control channel base station(s) on a fairly regular basis. The subscriber control packet payloads include a variety of information that is useful in achieving robust system operation. For example, the subscriber control packet payloads may include location information, lists of frequency pairs defining channels provided by the trunking site, and lists of frequency pairs defining control channels for other trunking sites that provide service in a geographical area (i.e., “neighboring” or “adjacent” trunking sites), among other things.
Each subscriber radio monitors the radio frequency (RF) signal conditions of the control channel of the trunking site with which the subscriber radio is currently registered (the “current trunking site”), and also occasionally samples the RF signal conditions of other control channels provided by any neighboring trunking sites of which the subscriber radio is aware. When the RF conditions for the current trunking site fall outside of acceptable ranges or when the subscriber radio fails to receive a subscriber control packet within a certain time period from receiving a last subscriber control packet, the subscriber radio may leave the current trunking site and attempt to register with a neighboring trunking site.
In a simulcast trunking site, the multiple base stations of the various sub-sites are connected to a control infrastructure that is configured to ensure the simultaneous communication of information by corresponding base stations of the various sub-sites, among other things. For example, the control infrastructure sends control messages (referred to below as “base station control packets”) to the control channel base stations, which indicate launch times for the control channel base stations to transmit the subscriber control packets, along with the payloads to be transmitted in the subscriber control packets. Because simultaneous transmission is particularly important in a simulcast trunking site, when a particular control channel base station does not receive a base station control packet indicating a launch time for a next subscriber control packet (e.g., when the control channel base station becomes isolated from the control infrastructure for some reason), the control channel base station simply refrains from transmitting a subscriber control packet. In a simulcast trunking site, this avoids potential destructive interference that may otherwise result from unsynchronized transmissions with the other control channel base stations of that trunking site.
The above-described protocol typically provides robust and seamless communication between a group of subscriber radios and the trunking sites with which they communicate. However, in certain situations, multiple ones (e.g., up to all) of the control channel base stations of a simulcast trunking site may become isolated from the control infrastructure and thus may abruptly stop transmitting subscriber control packets. In response to their failure to receive an anticipated subscriber control packet in adequate time (or of sufficient RF quality), the subscriber radios that are registered with that current trunking site may “scatter” to neighboring trunking sites (i.e., abandon the current trunking site and attempt to register with a neighboring trunking site).
When the control infrastructure recognizes that a base station isolation event is imminent and the control infrastructure has sufficient time to react, the control infrastructure may mitigate the situation. For example, during normal operation, a trunking site's registration hold off timer (which affects the timing of incoming registration requests) is zero. To mitigate an upcoming base station isolation event, the control infrastructure may instruct neighboring trunking sites to adjust their registration hold off timers to non-zero values. A typical trunking site is able to handle only a small number of registration requests per second. Accordingly, adjustment of a neighboring trunking site's hold off timer to a non-zero value enables the trunking site to stagger the timing of registration requests that may be received from a group of scattering subscriber radios.
However, when the control infrastructure is not able to recognize an imminent base station isolation event or to react in sufficient time (e.g., by instructing the neighboring trunking sites to adjust their hold off timers), the scattering subscriber radios may overwhelm the neighboring trunking sites with registration requests. Besides the service interruptions experienced by the scattering subscriber radios, the flood of registration requests may interfere with the neighboring trunking sites' ability to service subscriber radios that are currently registered with the neighboring trunking sites. Accordingly, sufficiently-persistent or unanticipated isolation of control channel base stations from the network infrastructure may result in widespread service interruptions with many subscriber radios. Such widespread service interruptions may take a significant period of time to overcome (e.g., many seconds or minutes). Therefore, there is a need for methods and apparatus for more responsively and robustly responding to control channel base station isolation from a control infrastructure.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present disclosure.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.